1. Field of the Invention
The invention relates to apparatus for feeding pulverised coal into an air line to a blast furnac, and a method of feeding pulverised coal using such apparatus. The apparatus comprises a plurality of feed tanks for the pulverised coal which are sequentially emptied into the air line and while not being emptied are filled again.
2. Description of the Prior Art
Fuel is fed into the hot blast introduced at the tuyeres of a blast furnace, in order to increase the heat content of the hot blast. Blast furnaces require a large amount of fuel, and therefore a low price fuel should be used. Oil is convenient to inject, but is expensive. Pulverised coal is an alternative but poses many difficulties in handling, particularly in view of its explosive nature in contact with oxygen.
An earlier patent describing a pulverised coal injection system is U.S. Pat. No. 3,306,238, but this does not discuss how the coal injection rate should be controlled. U.S. Pat. No. 3,720,351 discloses a system having three delivery tanks, which are emptied sequentially into the air blast line. Weight measuring transducers are provided to sense the weight of these tanks, and the differential of these weights with respect to time are calculated to provide a measure of the flow rate of coal to the blast furnace.
The use of three delivery tanks, which are generally supplied from a common storage tank, is advantageous, since it allows one tank to be emptying into the air blast line, one tank to be filling, and one tank to be in readiness to empty. If one of the tanks must be taken out of service for repair or maintainence, the remaining two tanks can still maintain a constant delivery though with no security against failure of one of them.
EP-A No. 0 059 904 also describes a system of feeding coal into metallurgical furnaces in which the weight of a feed tank is measured and differentiated with respect to time to give a flow rate.
In modern blast furnace practice, it is considered important to maintain an accurately controlled flow of the fuel into the air blast, and to avoid if possible substantial deviations in this flow rate. The present invention is particularly concerned to provide a control system and method which will increase the precision of the supply of pulverised coal into the air blast.